A striking new finding of mitochondrial hypertrophy with proportionate development of internal structure has been encountered in ischemic and contol gerbils treated with pentoxifylline, a drug considered to reduce cerebral edema and to be of benefit in patients with cerebral infarction. This xanthine-related compound is believed to affect the 3'-5' AMP system. It is proposed to delineate further the morphological changes and correlate them with chemical parameters related to oxidative metabolism and to energy production and utilization in gerbils and other animals.